Optimized Multi-Shell Engineering for Enhanced Luminescence in Core@Shell@Shell Upconversion Nanorods for Dual-Channel Imaging

Abstract

In this study, core@shell (NaYF₄:Yb,Er@NaYbF₄) and core@shell@shell (NaYF₄:Yb,Er@NaYbF₄@NaYF₄:Yb,Tm) upconversion nanorods (UCNRs) were synthesised using a seeded growth strategy. The NaYF₄:Yb,Er core, with a diameter of 129±4 nm, served as a template for epitaxial growth of subsequent shells. Structural and imaging studies confirmed the precise shell formation, enhancing light-harvesting capabilities, particularly in the near-infrared range, thereby facilitating superior energy transfer between the layers. Power-dependent upconversion luminescence (UCL) studies demonstrated the presence of two-photon and three-photon upconversion processes, leading to emissions across the red, and green regions, ideal for bioimaging applications. Optimisation of the first-shell thickness (15.5 nm) significantly enhanced the red and green emissions by 1.3 and 1.1 times respectively, while the core@shell@shell design further boosted these emissions by 2.3 and 1.5 times respectively, reducing surface quenching and improving luminescence stability. These UCNRs were tested in vitro using WRL-68 cells, maintaining over 95% cell viability up to 100 µg mL-1, demonstrating excellent biocompatibility. The internalisation of the UCNRs by WRL-68 cells was confirmed via fluorescence imaging, which captured strong red and green fluorescence signals, highlighting their potential as effective dual-channel bioimaging agents. These findings suggest that the optimised UCNRs are promising candidates not only for high-efficiency bioimaging but also for applications in photocatalysis and photonic devices, owing to their enhanced UCL and stability.

Keywords: Upconversion nanoparticles, Upconversion luminescence, core@shell nanoparticles, imaging studies, biomedical applications